1. Field of the Invention
The present invention relates to a solid electrolytic capacitor using a conductive polymer as a solid electrolyte layer.
2. Description of the Background Art
In connection with size and weight reductions of electric equipment in recent years, a small capacitor of a large capacity for high frequencies, which has a low impedance in a high frequency region, has come to be required.
Though a mica capacitor, a film capacitor, a ceramic capacitor, and the like are used as capacitors for high frequencies, these capacitors are not suitable for large capacities.
On the other hand, an aluminum electrolytic capacitor, a tantalum solid electrolytic capacitor and the like are suitable for large capacities. Though a large capacity can be attained at low cost with the aluminum electrolytic capacitor, there are problems such as a change of electrolyte solution used over time because of evaporation and a high impedance at a high frequency.
Since the tantalum solid electrolytic capacitor uses solid manganese dioxide as an electrolyte, degradation in capacity of the capacitor is small. The solid electrolyte of the tantalum solid electrolytic capacitor, however, is formed by impregnation of sintered tantalum with aqueous solution of manganese nitrate followed by pyrolysis of manganese nitrate at about 350° C., which impregnation and pyrolysis steps usually have to be repeated for several to tens times. Therefore, considerable labor is required in a formation process of the solid electrolyte. In addition, since a coating of manganese dioxide is deficient in self-repairing, it has a drawback such as low durability.
In order to solve problems mentioned above, use of a conductive polymer, which has a good electrical conductivity and is easily formed to be a solid electrolyte, as a solid electrolyte layer has been proposed recently (see Japanese Patent Laying-Open No. 60-037114 and No. 60-244017). With this technique, a solid electrolytic capacitor can be obtained which, in comparison with the aforementioned aluminum electrolytic capacitor, tantalum solid electrolytic capacitor and the like, requires lower manufacturing cost, ensures a capacitance, has a dielectric coating that is not damaged, and has a small leaked current.
The conductive polymer is formed by electrolytic polymerization of heterocyclic monomers such as pyrrole, thiophene and furan with a supporting electrolyte to form a film of a polymer having a good conductivity on the dielectric coating using an anion of the supporting electrolyte as a dopant. As the dopant of the conductive polymer, a perchlorate ion, a boron tetrafluoride ion, a para-toluenesulfonate ion, a dodecylbenzenesulfonate ion, or the like is used.
A chemical oxidative polymerization method, an electrolytic oxidative polymerization method or the like is used as a method of forming the solid electrolyte layer including the conductive polymer containing the aforementioned dopant in the solid electrolytic capacitor as such. The chemical oxidative polymerization method is a method of performing oxidative polymerization of monomers using an oxidizing agent, and the electrolytic oxidative polymerization method is a method of performing oxidative polymerization of monomers on an anode utilizing an oxidation reaction occurring at the anode during electrolysis. Though the conductive polymer formed by the electrolytic oxidative polymerization generally has higher strength and conductivity than those of the conductive polymer formed by the chemical oxidative polymerization and forms a uniform good-quality solid electrolyte layer, it has a low heat resistance and when exposed to a high temperature, desorption of the dopant occurs and an ESR (Equivalent Series Resistance) increases.